Underwater electrodynamic acoustic transducer with air-filled composite diaphragm



Aprll 4, 1961 T, HARms 2,978,669

UNDERWATER ELECTRODYNAMIC ACOUSTIC TRANSDUCER WITH AIR-FILLED COMPOSITE DIAPHRAGM Filed March 8, 1954 h-WWW ATTC R N EY5 Wilbur '1. Harris, Southbury, Conn., assignor to The Harris Transducer Corporation, Southhury, Conn., a corporation of Connecticut Filed Mar. 8, 1954, Ser. No. 414,603 5 Claims. (Cl. 340-8) My inventionrelates to improved electroacoustic transducer means and in particular to electromagnetic transducers,as of the character described in greater detail in my co-pending patent application Serial No. 241,470, filed August 11, 1951.

It is an object of the invention to provide an improved transducer of the character indicated. 7

It is another object to provide an improved means for coupling a transducer of the character indicated to the medium into which the transducer is to radiate or is otherwise to respond.

It is a further object to provide means for vastly improving the radiating efiiciency of transducers of the character indicated.

It is a specific object to provide improved diaphragm means for coupling a movable transducer element to a radiative medium.

Other objects and various further features of novelty and invention will be pointed out or will occur to those skilled in the art from a reading of the following specification in conjunction with the accompanying drawings. In said drawings, which show, for illustrative purposes only, preferred forms of the invention:

Fig. 1 is a fragmentary sectional view of a transducer incorporating features of the invention; and

Fig. 2 is a similar view illustrating a modification.

Briefly stated, my invention contemplates employment of novel diaphragm means in conjunction with the movable element of an electroacoustic transducer for providing improved impedance-matching characteristics, as for underwater use, so that a more efiicient energy transfer may be possible. between the moving transducer element and the radiating medium. Such diaphragm means may be of composite or unitaryconstruction, but essentially comprises the utilization of a uniformly thick blanket, slab, or pocket of air interposed .between the driving diaphragm and the radiating medium so that the Patented Apr. 4, 1961 alternate for adjacent coils so that strong fields may be established in the gaps between adjacent pole pieces, as will be understood. In order to provide a more rugged assembly, I show fillings of plastic material, as at 22, between turns of adjacent coils so that strong fields may be established in the gaps between adjacent pole pieces, as will be understood In order to provide a more rugged assembly, I show fillings of plastic material, as at 22, between turns of adjacent coils (19-20) in the same coil slot between adjacent pole pieces (IS-16).

Asjexplained more fully in'the said co-pending application, the electrodynamic strips supported in the respective flux gaps are the driving vibratile elements of the transducer. Thus, a first coil may include conductive strips having a first leg or length'24 in the gap 11 and a second leg or length 25 in the gap 12, and I have shown my preference that the coil 2425 be developed helically. from strip material laminated and consolidated into a single rugged vibratile element, the views in Figs. 1 and 2 being sectional through opposite. legs of the same integral coil structure. The loop or coil (for which lengths 2425 constitute the opposite legs) may include end connections (not shown) to complete the coil, as will be understood. A second coil comprising a first leg or length 27 and another leg or length (not shown) may be of similar construction and may be supported in the next pair of flux gaps. F

In accordance with the invention, I provide novel diaphragm means for efi'lciently coupling the, energy available in the moving coils 2425 and 27 tothe medium into which energy is to be radiated. Such diaphragm means essentially comprises a driving surface which may rigidly support the respective electrodynamic coils 24- 25 and 27, and an air-filled blanket interposed between the radiating face and the medium and thus constituting a compliant coupling.

In the form shown in Fig. 1, the driving surface is part of a driving diaphragm member 30 to which the various legs of the moving coils are rigidly secured through suitable insulating means (not shown). I have schematically shown this secured relationship by means of bolts, as at 31. The'radiating surface'may be the interface between the air blanket and the water; but, for convenience, this interface is defined between an acoustically transparent seal 32 and the air pocket, which,

. inthe form shown, is'a uniformly thick slab of air-filled air may serve to couple the vibrating diaphragm to the water. In'one form tobe described, the blanket of air is provided by yieldable air-filled material, such as' foam rubber, carried by the radiatingside of driving diaphragm and appropriately sealed against water so as to preserve a homogeneous distribution of air. In the other form to be described, a pure air space is provided between the diaphragm and an acoustically transparent 'sleal member, but the impedance-transforming function V of the air. belt or pad is in both cases the same.

Referring to Fig; l of the drawings, my invention is shown in application to an electroydynamic transducer comprising a magneticEcore'means 10, 'which may be a j 3 single block of-magnetic-core material, providing a grid of substantially parallel flux gaps '11--12 13 between spaced pole'pieces 14+15-16-17. The core 10 may be permanentlymagnetized, but I show a plurality of coils for polarizing the same. This, a first coil 19 may be (linked to the pole piece 15 and further coils 20-21 to the pole pieces 16,- '17, respectively. -'The polarity of i polarization etfected by h r1s 19-;-2c- 21 should material, such as foam rubber 33. I prefer that the described parts shall'be bonded, and find it convenient to interpose a thin layerIi t of solid rubber or the like between the blanket 33 and the diaphragm 30, but since 'both layers 32 and 34 are of acoustically transparent material, that is, having the sound-transmitting properties of water or other medium into which radiation is to be effected, the air pocket defined by the blanket 33 is effectively interposed between the medium andthe diaphragm 30.

As a matterof structural detail, I employ the rubberlike sheath 34 as a means for supporting or transversely locating the diaphragm .30;"therefore, the sheath 34 is shown strengthened by added thickness at the edges, as at 35, and is securely cla'mpedby bolt means 36 'anda clamping strip 37 to the main frame? or housing 38. ofthe transducer. The air-filled blanket :33 may next be placed ,over the sheath 34, and finally the sealing sheet 32"rnay be secured by bolt means 39 to the side frame of the housing. A thin metal difiusion barrier 53 is desirable to prevent loss of air content in the cellular rubber when thetransducer is subjected to pressure fora long time.

To complete the structure, 'a boot or jacket 4a of rubber v like sealing materialinay cover all exposed metal parts.

' fAs explainedinthe said co-pending application, my transducer may be renderedless depth-dependent by employment of a pressure-compensating region 41 in the space behind the magnet structure and preferably parallel to and uniformly spaced therefrom. Local apertures 42 through the magnet structure may permit sufiicient communication between the compensated region 41 and the air space in which the coils 24- 25 and 27 move. The compensator is shown to comprise a flexible bag 43 secured peripherally at backplate 44 of the housing and with one or more ports 45 to permit free-flooding the bag 43.

In use, the air pocket constituted by the blanket 33 permits the diaphragm 30 and the coil structures to resonate or to approach resonance at relatively large amplitude, and the compressional excursions within the air pocket react with oscillating bodily displacements of the local volume of water immediately overstanding the face of the transducer. Mathematically, the ratio of compliances per unit area of driven blanket surface and water surface, respectively, is approximately 8 dw Ps E pw 'i where d is the effective thickness of the air slab 33 and a' is the effective thickness of the resonated water mass, where p and p are the densities of the air slab and of water, and where C, and C are the sound velocities in the two media.

At resonance, the mass of the diaphragm and coil assembly, the compliant air slab, and the reactive mass component of the acoustic impedance of the medium constitute a mechanical oscillating system in which the ratio of the velocities is the inverse of the associated masses. At frequencies above resonance, the water mass is not effectively driven, and the device becomes ineflicient. At frequencies below resonance, the air slab is an etfective pressure transmitter, with velocity transformation, and the transducer has a broad frequency range in which efliciency is increased, as compared to direct drive without the air slab.

The function of the transmitting pad 33 appears to be directly analogous to that of a coupling transformer, and at frequencies above resonance the shunt capacity of the transformer causes a cutofi. At resonance, there may be more or less of a response peak, but below resonance is a fiat response in which ideal transformer action is realized. Finally, at very low frequencies, there is a substantial cutofi when flux linkage has become inadequate and time-differentiation occurs, corresponding to inadequate slab thickness.

In Fig. 2, I show a modification in which the air-filled blanket, pocket, or pad is literally an unobstructed air volume 50 between a driving diaphragm S1 and a sealing diaphragm 52. For purposes of minimizing air and water diffusion through the rubber, the diaphragm 52 may in corporate a thin, effectively transparent, metal membrane 53 which I have shown terminating short of the side of the transducer 'so' as not toimpair acoustic transparency of the seal 52. The magnet 10 and other driving parts of the structure may be as described for Fig. l and have therefore been given the same reference characters. The diaphragm 51 may be suspended by a continuous sheath, as at 34 in the case of Fig. l; but I have shown a flexible lip 54 secured to the edge or rim of the housing 55, and accurate lateral location of the diaphragm 51 is achieved by a sheet-spring web. 56 bolted at 57 to the diaphragm 51 and at 58 to the housing 55. Operation will be as described for the structure of Fig. 1. For both cases, the air pocket or slab functions"? to provide the same transformer action, pressure equalization in the air slab being achieved by one or more small apertures 42 in diaphragm 51.

It will be appreciated that I have described means for more efficiently transmitting energy from a mechanically vibratile element into a radiating medium. The compliant slab constituted by the air pocket or air blanket, as the case may he, provides an impedance-matehing action which maximizes the force per unit area which can be exerted. As a result, efficiency, compactness, and cost are favorably affected.

While I have described the invention in detail for the preferred forms illustrated, it will be understood that modifications may be made within the scope of the invention as defined in the claims which follow.

I claim:

1. In an underwater transducer of the character indicated, a movable driving diaphragm, a plurality of electro-magnetic elements in driving relation with one side of said diaphragm, a substantially uniform-thickness blanket of air-filled material substantially coextensive with said diaphragm on the other side thereof, and acoustically transparent means sealing said blanket against water exposure.

2. A transducer according to claim 1, and including a pressure-compensating bag substantially co-extensive with said diaphragm and on the side of said driving elements away from said diaphragm, the air behind said diaphragm being in communication with the air over said bag, and a free-flooding fluid communication between the exterior of said transducer and the outer side of said bag.

3. In an underwater transducer of the character indicated, magnetic-core means providing a plurality of spaced elongated gaps, electro-dynamic coil means comprising a movable strip of conductive material including opposed lengths in the respective magnetic fields of two of said gaps, diaphragm means including a driving surface rigidly supporting said conductive lengths, and a cornpliant radiating surface on said diaphragm means and comprising a substantially uniform-thickncss air-filled volume.

4. A transducer according to claim 3, in which said air-filled volume is defined by a blanket of air-filled material.

5. A transducer according to claim 3, in which said air-filled volume is defined between said diaphragm and a resilient sealing member substantially coextensive with said diaphragm and spaced therefrom.

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